Major improvements:
- Split tests into 4 parallel groups (store, server, plugin, other)
  * 3-4x faster test execution (4-6min → 1-2min)

- Enable golangci-lint cache (was disabled)
  * Saves 20-30s on lint checks

- Remove unnecessary flags
  * check-latest: API call overhead
  * --verbose: unnecessary output
  * skip-cache: disabled caching

- Add race detector (-race)
  * Better concurrency bug detection
  * Only 10-20% overhead

- Add coverage tracking
  * Per-group coverage upload to Codecov
  * Better visibility into test quality

- Add concurrency control
  * Cancel outdated PR runs
  * Saves runner minutes

- Clean up output processing
  * Remove test.log and complex awk/sed parsing
  * GitHub Actions shows output by default

Performance impact:
- Setup: 30s → 10s (2-3x faster)
- Tests: Sequential → 4 parallel jobs (3-4x faster)
- Total: 4-6min → 1-2min (~70% faster)